Known ceiling mount detectors include a single Fresnel lens and one or more passive infrared (PIR) sensors, such as a dual element pyroelectric sensor. For example, FIG. 1A is a front view of a known Fresnel lens 100 used in known ceiling mount detectors, FIG. 1B is a perspective view of the known Fresnel lens 100 used in known ceiling mount detectors, FIG. 1C is a front view of the dual element pyroelectric sensor 150 used in known ceiling mount detectors, and FIG. 1D is a perspective view of the dual element pyroelectric sensor 150 used in known ceiling mount detectors. As seen in FIG. 1A and FIG. 1B, the known Fresnel lens 100 used in known ceiling mount detectors can include a full, single lens with fully concentric annular sections.
However, known ceiling mount detectors can only detect an alarm condition in some directions. Indeed, pyroelectric sensors are non-rotationally symmetric, causing a look down area of a detector to be non-rotationally symmetric. Because of differences in performance of the pyroelectric sensors in some directions, known ceiling mount detectors miss detecting alarm conditions in a vertical direction. Specifically, the dual element pyroelectric sensor only outputs a strong and clear signal indicative of the alarm condition when energy from the alarm condition, such as an intruder, is detected by a first element of the sensor and then by a second element of the sensor.
For example, as seen in FIG. 2A, the dual element pyroelectric sensor in a known ceiling mount detector 200 projects a positive detection area 210 and a negative detect area 220 into the look down area on the ground. As seen in FIG. 2B, when the intruder I passes from one area 210 to the other area 220 in a direction along a strong X axis (considered to be 90° relative to the sensor), the dual element pyroelectric sensor will output a strong signal, as illustrated on a chart 250 shown in FIG. 2C. However, as seen in FIG. 2D, when the intruder I passes evenly through the positive and negative areas 210, 220 in a direction along a weak Y axis (considered to be 0° relative to the sensor) or does not pass from one area to the other area, the pyroelectric sensor will not output a signal, as illustrated in a chart 260 shown in FIG. 2E, or will output a signal too weak to be detected as indicative of the alarm condition. As discussed herein, quad element pyroelectric sensors have similar limitations.
In view of the above, there is a continuing, ongoing need for an improved ceiling mount detector.